Power semiconductor devices are used in power converting devices, such as converters and inverters with high efficiency and low power consumption, and are indispensable for controls of rotary motors or servo motors. The power control device requires the characteristics of low loss, low power consumption, a high-speed operation, high efficiency, and no environmental problem, that is, no adverse influence on the surroundings. In order to meet the demand for a power control device with low loss and high efficiency, a diode with a broad buffer structure has been proposed as an improved type of the diode used in the power control device. The broad buffer structure means a structure in which the impurity concentration distribution of an n− drift layer has a peak (local maximum value) in the vicinity of a center portion of the n− drift layer and which has a broad buffer region including a region in which the impurity concentration distribution is inclined so as to be reduced toward an anode and a cathode.
The diode with the broad buffer structure allows a reduction in the emitter injection efficiency of the related art and realization of soft recovery characteristics and an oscillation prevention effect in a high-speed operation (for example, carrier frequency: 5 kHz or more) which has been difficult to be achieved in the lifetime distribution control technique.
A method using a hydrogen-induced donor has been proposed as a method of manufacturing the diode with the broad buffer structure. In the method, an floating zone (FZ) bulk wafer is irradiated with protons (hydrogen ions, H+) such that the protons H+ reach the depth of the n− drift layer which has been hardly achieved by a general n-type doping element (phosphorus or arsenic) ion injection method, thereby forming a lattice defect, and a heat treatment is then performed. In the method, the irradiation with the protons and the heat treatment cause a donor (for example, called a hydrogen-induced donor or a hydrogen-associated donor) to be formed in the vicinity of the range Rp of the proton H+ in the wafer by a defect complex including the proton H+ (for example, see the following Patent Literature 1 (Paragraphs 0020 and 0021) and the following Patent Literature 2 (Abstract)). In addition, a method has been proposed in which oxygen is introduced into a wafer and is combined with the hydrogen-induced donor, thereby forming a high-concentration broad buffer region (for example, see the following Patent Literature 3 (Paragraph 0011)).
In general, a silicon (Si) power semiconductor, an FZ wafer which is cheaper than an epitaxial wafer is used to manufacture an IGBT or a diode from an economic viewpoint. In addition, it is known that a method which irradiates a silicon wafer with neutron beams to convert silicon into phosphorus (P), which is a stable isotope, using nuclear transmutation, thereby forming phosphorus, which is an impurity, in a wafer (hereinafter, referred to as a neutron irradiation wafer) is effective in uniformly distributing impurities in the wafer. The resistivity variation of the neutron irradiation wafer, for example, a 6-inch wafer is about ±8%.
As a method of forming the neutron irradiation wafer, a method has been proposed which changes protons H+ into donors using irradiation with the protons H+ and a heat treatment and injects donors with a concentration more than that of the wafer before neutron irradiation into the n base region (n− drift layer) (for example, see the following Patent Literature 4).